1. Field of the Invention
The present invention relates to an alignment apparatus used for a projection exposure apparatus including a projection optical system for transferring a pattern formed on a mask to a photosensitive substrate, and more particularly to an alignment apparatus of a heterodyne interference method for relative alignment of a mask with a photosensitive substrate by emitting alignment light having a plurality of wavelengths to the photosensitive substrate.
2. Related Background Art
In a projection exposure apparatus used for producing semiconductors, liquid crystal display elements, and so on, multilayer circuit patterns are formed on a wafer (or a glass plate, etc.). It is necessary for the multilayer circuit patterns to register with each other with high accuracy. In this case, it is very important to align a reticle (or a photo mask and the like) with the wafer with high accuracy, particularly when a pattern formed on the reticle is to be transferred to a second or subsequent layer on the wafer. Therefore an alignment system is necessary for detecting the relation between a reticle position and a wafer position with high accuracy. Thus, there have been used an alignment system of a TTL (Through-The-Lens) type for detecting a relatively positional relation between the reticle and the wafer through a projection optical system.
It is necessary to use light having a different wavelength from exposure light as alignment light for use in an alignment system in order not to expose a photoresist coated on the wafer.
However, employment of alignment light having a wavelength different from a wavelength of exposure light causes chromatic aberration in projection optical systems. U.S. Pat. Nos. 5,100,237 and 4,492,459 propose ideas to solve the problem of chromatic aberration.
In a process disclosed in U.S. Pat. No. 5,100,237, arrangement of a chromatic aberration correcting lens about an optical axis of a projection optical system at an entrance pupil position enables the chromatic aberration between the wavelength of exposure light and that of alignment light to be corrected, thereby detecting (.+-.) primary diffracted light beams from a wafer mark to perform an alignment between a reticle and a wafer.
In a process disclosed in U.S. Pat. No. 4,492,459, arrangement of a correcting optical system outside or inside an exposure optical path between a reticle and a projection optical system enables chromatic aberration caused by alignment light passing through a projection optical system to be corrected. A reticle mark image and a wafer mark image formed on a wafer are detected through the projection optical system before performing an alignment.
The applicant of this application has proposed an alignment apparatus of a heterodyne interference method with respect to correction of chromatic aberration relative to alignment light. In the alignment apparatus of a heterodyne interference method, arrangement of chromatic aberration controlling members such as phase gratings to a position adjacent to a pupil plane of a projection optical system permits alignment light to be deflected, thereby performing alignment of a reticle and a wafer with each other with high accuracy.
In addition to the problem of the chromatic aberration, since detection of a position of a wafer mark by a monochromatic light weakens a reflected light depending on a photoresist thickness, unevenness of a wafer mark, and so on, it is often difficult to detect the wafer mark. Therefore, the applicant of this application also has proposed an apparatus in which light beams having a plurality of wavelengths (colors) are used as alignment light beams and phase gratings for chromatic aberration are arranged relative to the light beams of wavelengths, respectively to thereby perform alignment with more high accuracy.
As mentioned above, according to the apparatus in which light having a plurality of wavelengths is employed as alignment light and the phase gratings are arranged adjacent to a pupil plane of a projection optical system and in facing relation to the pupil plane, since chromatic aberration by the projection optical system itself offsets that by the phase gratings disposed adjacent to the pupil plane of the projection optical system, resulting chromatic aberration is negligible in a plurality of wavelengths of alignment light. Strictly speaking, however, since some chromatic aberration remains, plurally colored interference fringes can not be aligned with each other simultaneously and precisely on both the reticle and the wafer.
Movement of a viewing system (or a microscope) of an alignment apparatus within an exposure area causes an amount of deviation between plurally colored interference fringes relative to the alignment light having a plurality of the wavelengths to be changed because of the characteristics of a projection optical system. Multilayer circuit patterns are transferred to a wafer by exposing it. Each of the transferred multiple layers has a different reflectance with respect to a wavelength. For example, a layer does not greatly reflect light having one wavelength, another layer does not reflect light having another wavelength. In this case, changing the amount of deviation between the two-colored interference fringes as mentioned above causes an alignment detecting position to be deviated by the change of the amount of deviation between the two-colored interference fringes in comparison with a layer uniformly reflecting the two light beams having the difference wave-lengths. In order to solve the problem, a process (or a contrast method) can be used for adjusting an optical system to maximize a contrast of a beat signal (or a wafer signal) obtained from a wafer mark to decrease the deviation between the two-colored interference fringes. However, it is necessary for the amount of deviation between the two-colored interference fringes to be controlled within a range of from several "nm" to several tens "nm" in order to restrict the deviation therebetween to the extent that it can be made negligible. Thus, the contrast method is not sufficient.
Even if deviation between the two-colored interference fringes relative to the two-colored light on the wafer is adjusted to be decreased, adjustment thereof automatically causes deviation between the two-colored interference fringes to be generated on a reticle. For example, when a ratio of light quantity of the two-colored, light is 1:1 and the one-colored interference fringe is deviated from the other colored interference fringe by a half pitch, a beat signal (or a reticle signal) is not generated from a reticle mark.